The role of the pineal gland hormone melatonin in the regulation of estrogen receptor (ER) activity and the growth of human breast cancer cells is being studied in vitro and in vivo. We have shown that melatonin rapidly increases cytosolic and nuclear ER activity of human breast cancer cells, increases the binding of ER to DNA cellulose but reduces binding to deoxyguanylic acid. The increased ER activity is not extractable, and is not associated with a change in sedimentation rate on sucrose density gradients or molecular weight PAGE. Melatonin has been found to decrease 3H-thymodine incorporation and alter cell growth of human breast cancer cells. We are currently determining the conditions which effect melatonin alteration of growth of human breast cancer cells in vitro and in vivo. We have also studied the secretion of plasma melatonin in women with breast cancer, women at high risk for breast cancer, and normal subjects. We found that women with ER or PR positive tumors have a lower plasma level of melatonin than ER or PR negative patients of normal subjects. The pattern of secretion of women at high risk is similar to that of breast cancer patients and normal subjects, suggesting that the plasma levels of melatonin are important in determining the hormone dependency of human breast cancer. We have also characterized the ER from variant human breast cancer cell lines and from patients using PAGE and a radiolabel, tamoxifen aziridine, which binds covalently to the ER. We have found that the molecular weight of the basic ER subunit structure is 62,000 dalton. Several molecular weight species have been identified in the cytoplasm of these cells, and the pattern of this distribution varies according to whether the cells are hormone responsive or nonresponsive. We are presently determining the significance and the characteristics of these new molecular weight species.